The effect of frequency and phase on the threshold of squid axon space-clamped in a double sucrose gap and stimulated by AC sinusoidal current will be studied. The input-output relationship between the applied sinusoidal current stimulation and the voltage response (a distorted sinusoid, due to membrane rectificaton, with a phase-locked spike superimposed) will be investigated. In preliminary experiments it has already been established that the response occurs without lead or lag at the membrane's resonant frequency. At lower frequency the response leads, at higher frequencies it lags. An attempt will be made to determine whether a threshold voltage or a threshold quantity of electricity is the criterion for excitation, and whether it is the frequency of stimulation which determines whether it is the one or the other. Results will be compared with the theoretical axon computed according to the HH equations. In addition, stimulation by sinusoidal depolarizing pulses separated by silent periods of zero potential of varying duration will also be used to investigate the possible effect of previous hyperpolarization in the first type of stimulation. The effect of varying the calcium ion concentration and of temperature upon the phenomenon will also be studied. Possible significance with the general problem of phase locking and of encoding of information will be investigated.